This invention relates to an optical harmonic generating device for use in the optical information processing field utilizing coherent light or in the measurement control field applying light.
A conventional optical harmonic generating device was disclosed, for example, by T. Taniuchi and K. Yamamoto in "Second generation with GaAs laser diode in proton-exchanged LiNbO.sub.3 waveguides," ECOC '86, TuC5, 1986. Referring first to FIG. 1, harmonic generation (wavelength 0.42 .mu.m) with respect to light of a fundamental wave with wavelength of 0.84 .mu.m is described below. When light of a fundamental wave P1 enters a plane of incidence of a buried type optical waveguide 2, if the conditions are sufficient to equalize the effective index N1 of the guide mode of the fumdamental wave and effective index N1 of a harmonic wave, the harmonic wave P2 is efficiently emitted from the optical waveguide 2 into a LiNbO.sub.3 substrate 1 so that the overall device acts as an optical harmonic generating device.
Such a conventional optical harmonic generating device requires a buried type optical waveguide as a fundamental constituent element. The method of fabricating this buried type optical waveguide was, as disclosed by J. L. Jackel, C. E. Rice, and J. J. Veselka in "Proton exchange for high-index waveguides in LiNbO.sub.3," Appl. Phys. Lett., Vol. 141, No. 7, pp. 607-608, 1982, as follows. Cr or Al is evaporated onto a substrate made of LiNbO.sub.3 which is a ferroelectric crystal, and a slit of several micrometers in width is formed by a photo process and etching, and the substrate is heated in benzoic acid to form a high refractive index layer (difference in refractive index from that of the substrate of about .DELTA.Ne=0.13).
Turning to FIG. 2, a conventional method of fabricating a buried type optical waveguide by employing the proton exchange technique in a solution is explained below. By heating the LiNbO.sub.3 substrate 1 on which protective mask 4 and slit 5 are formed in benzoic acid 6, exchange of H.sup.+ protons in benzoic acid 6 and Li.sup.+ in the LiNbO.sub.3 substrate occurs only beneath the slit 5, and a high refractive index layer 2 composed of H.sub.x Li.sub.1 --.sub.x NbO.sub.3 (0.ltoreq.x.ltoreq.1) is formed. In FIG. 3C, after removing the protective mask 4, the surface perpendicular to the depth of the buried type optical waveguide 2 is optically polished, and by introducing a fundamental wave, a harmonic wave can be taken out.
The fabricating process is further described with reference to FIG. 3. In FIG. 3A, a protective mask 4 is shown as formed on the LiNbO.sub.3 substrate 1 by an ordinary photo process. The material of this protective mask 4 is aluminum. In FIG. 3B, the substrate is heated for 12 minutes in benzoic acid (230.degree. C.), and a 0.5 .mu.m thick buried type optical waveguide 2 is formed. In FIG. 3C, after removing the protective mask 4, the surface perpendicular to the depth of the buried type optical waveguide 2 is optically polished, and by introducing a fundamental wave, a harmonic wave can be taken out.
The optical harmonic generating device fabricated by the above treatment with benzoic acid has a maximum conversion efficiency at a thickness of the waveguide 2 of 0.5 .mu.m for a fundamental wave P1=100 mW with a wavelength of 0.84 .mu.m, and a harmonic wave of P2=2 mW was obtained when the length of the waveguide was set at 10 mm. In this case, the conversion efficiency P1/P2 is 2%.
In the optical harmonic generating device having such a buried type optical waveguide, the refractive index difference in the lateral direction was small, and confinement of light in the lateral direction was weak, and the width of the actually fabricated optical waveguide expanded in the lateral direction as compared with the mask width to further worsen the confinement. It was hence difficult to obtain a conversion efficiency of over 5% which is a practical level of efficiency of an optical harmonic generating device.